1. Field of the Invention
The present invention relates to a power semiconductor device, in particular, a power semiconductor device having a gate electrode embedded in a trench.
2. Description of the Background Art
Some power semiconductor devices are used as non-contact switches for controlling a large amount of power. Such devices with large capacities are applied to, for example, inverter circuits in household electric appliances such as air conditioners, refrigerators, and washing machines, each of which has been developed with emphasis on energy saving, and are also applied to motor control in trains such as Shinkansen bullet trains and underground railways. Further, in recent years, in consideration of global environment, power semiconductor devices have begun to be applied to use in controlling an inverter/converter of a hybrid car traveling using both an electric motor and an engine, and use in a converter for solar photovoltaic power generation or wind power generation. As such, the field of application of power semiconductor devices is getting wider.
As the power semiconductor devices described above, for example, an IGBT (Insulated Gate Bipolar Transistor) is exemplified. An IGBT is a representative switching element for controlling a large current while securing small loss.
Now, a principle of operations of the IGBT is briefly described.
Described first is turning-on. By applying a sufficient positive voltage (for example, +15V) between a gate and an emitter, a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) provided on the surface side of the IGBT is turned on. Accordingly, a region between a collector p+ layer and an n− drift layer is forward-biased to inject positive holes from the p+ layer to the n− layer. An amount of electrons corresponding to the amount of charge of the injected positive holes are concentrated in the n− drift layer to decrease the resistance of the n− drift layer (conductivity modulation). In this way, the IGBT is brought into the on state.
Described second is turning-off. When the gate-emitter voltage is decreased, the MOSFET provided on the surface side of the IGBT is turned off. Accordingly, the injection of positive holes from the collector p+ layer is stopped, resulting in depletion of the n− drift layer. Accordingly, positive holes having been already injected are discharged to the emitter side, thereby interrupting the current.
The decrease of resistance of the n− drift layer caused by the conductivity modulation in the on state means reduced resistance of the device, and the collector-emitter voltage on this occasion is called “on voltage”. The current obtained from the positive holes remaining upon the turning-off results in switching loss. Hence, as more positive holes and electrons (hereinafter, collectively referred to as “carriers”) are injected into the n− drift layer to achieve reduced resistance, the loss (switching loss) resulting from the carriers remaining upon the turning-off is increased. In other words, there is a tradeoff relation between the on voltage and the switching loss.
To remedy this tradeoff characteristic, a trench type IGBT is disclosed in which transistor cell density is improved using a microfabrication technique. The trench type IGBT has a gate electrode embedded in a trench, which is formed on a semiconductor layer, with a gate insulating film interposed therebetween. A technique for forming such a trench is disclosed in, for example, Japanese Patent Laying-Open No. 06-291178. In addition, apart from the IGBT, a CSTBT (Carrier Stored Trench-gate Bipolar Transistor), an IEGT (Injection Enhanced Gate Transistor), and the like have been developed in each of which carrier density in a drift layer is improved.
When unexpected events occur such as load short and arm short, a large current/high voltage is applied to the IGBT. Even in such a situation, the IGBT element needs to withstand up to a certain degree of energy. In the course of collector voltage increase and current attenuation caused by the gate turning off upon occurrence of the short circuit, carriers (positive holes) stored in the n− drift layer are drained corresponding to dv/dt, i.e., a time differential value of the collector-emitter voltage. If the positive hole currents flow through base regions of parasitic npn transistors of the MOSFET, the IGBT is likely to be latched up disadvantageously.
An exemplary technique for preventing such latch-up is disclosed in Japanese Patent Laying-Open No. 2008-021918. According to Japanese Patent Laying-Open No. 2008-021918, a semiconductor device includes a collector layer of a first conductive type; a semiconductor layer of a second conductive type; a base region of the first conductive type; an emitter region of the second conductive type; a first trench; a first gate electrode; a second trench; a second gate electrode; an emitter electrode connected to the base region and the emitter region; and a collector electrode connected to the collector layer. The semiconductor layer is formed on the collector layer. The base region is formed at a surface of the semiconductor layer. The emitter region is formed at a portion of a surface of the base region. In order to form the first trench, the surface of the emitter region is dug to reach the semiconductor layer. The first gate electrode is embedded in the first trench with a first insulating film interposed therebetween. In order to form the second trench, the surface of the base region other than the emitter region is dug to reach the semiconductor layer. The second gate electrode is embedded in the second trench with a second insulating film interposed therebetween. The second trench is deeper than the first trench.
According to the above-described technique of Japanese Patent Laying-Open No. 2008-021918, in addition to the first gate electrode essential to an IGBT, the second gate, an extra gate dedicated to prevention of latch-up, has to be provided. This greatly changes the structure of an IGBT, thus greatly varying electric characteristics of an IGBT.